Known driving mode switching apparatuses for a four-wheel-drive vehicle, which include a motor (i.e., power source), a reduction mechanism connected to the motor, a spring mechanism (i.e., standby mechanism) connected to the reduction mechanism, an output rod connected to the spring mechanism and which switch drive states of a four-wheel-drive vehicle, are described in JP3618112B and JP2001-71782A. For example, the driving mode switching apparatuses for the four-wheel-drive vehicle switch a two-wheel-drive state and a four-wheel-drive mode, by a displacement of the output rod.
Particularly, the known driving mode switching apparatus for the four-wheel-drive vehicle described in JP2001-71782A includes a temperature sensor for detecting the temperature of the motor, and the energization to the motor is limited when the temperature of the motor reaches a predetermined degree.
The level of the driving force required to displace the output rod of the driving mode switching apparatus (i.e., actuator) for the four-wheel-drive vehicle depends oil situations of an auxiliary transmission and/or a transfer apparatus for a drive train for a vehicle on which a clutch or a sleeve displaced by the output rod are mounted. Particularly, in case of immediately after starting the vehicle in the environment with low temperature, for example, early in the morning, lubrication oil may not be supplied to each portion inside the transfer apparatus and the auxiliary transmission (i.e., generation of a lack of lubrication) and/or the viscosity of the lubrication oil may be high. Under the aforementioned states of the transfer apparatus and/or the auxiliary transmission, in order to switch the driving modes of the transfer apparatus and/or the auxiliary transmission, the driving force required to the driving mode switching apparatus which displaces the sleeve or the clutch, or the like, increases. In other words, the driving force required to the driving mode switching apparatus is susceptible to the influence of the temperature of the auxiliary transmission and the transfer apparatus which is an object to be operated. And in order to stably displace the sleeve or the clutch, or the like, even at the low temperature, the motor of the driving mode switching apparatus needs to be increased in size and in output performance. Thus, there is a drawback that the driving mode switching apparatus per se is increased in size. Further, with the driving mode switching apparatus which includes the motor being increased in size and increased in output performance, there is a drawback that the power consumption when displacing the output rod in the normal temperature is increased.
According to the driving mode switching apparatus described in JP3618112B and JP2001-71782A, because the motor with high output performance is needed to be adopted, the cost for the motor increases, and the size of the driving mode switching apparatus per se increases.
In those circumstances, the temperature sensor for detecting the motor temperature described in JP2001-71782A is used for preventing the overheating of the motor, and is not applied for switching the driving modes of the four-wheel-drive vehicle. Further, because fluctuation rates of the motor temperature are large and the motor temperature fluctuates quickly, there is a risk of generating a phenomenon of, for example, overshooting and/or hunting at the motor output. Accordingly, the motor output is unlikely to be controlled appropriately.
A need thus exists for a driving mode switching apparatus for a four-wheel-drive vehicle which is not susceptible to the drawback mentioned above.